1. Field of the Invention
This invention relates to fiber optic sensors for measuring temperature and pressure and to systems based thereon.
2. Description of Related Art
Optical fibers generally include a cylindrical core, a concentric cylindrical cladding surrounding the core, and a concentric cylindrical protective jacket surrounding the cladding. The core is made of transparent glass or plastic having a certain index of refraction. The cladding is also made of transparent glass or plastic, but having a different, smaller, index of refraction. The ability of the optical fiber to act as a bendable waveguide is largely determined by the relative refractive indices of the core and the cladding.
The refractive index of a transparent medium is the ratio of the velocity of light in a vacuum to the velocity of light in the medium. As a beam of light enters a medium, the change in velocity causes the beam to change direction. More specifically, as a beam of light travels from one medium into another medium, the beam changes direction at the interface of the two media. In addition to changing direction at the interface of two media, a portion of the incident beam is reflected at the interface such that the energy of the beam traveling through the second medium is diminished (the sum of the energy of the refracted and reflected beams must equal the energy of the incident beam). The angles of reflection and refraction can be predicted using Snell's law if the refractive indices of both media are known.
By altering the indices of refraction of two adjacent media, the angle of refraction and the angle of reflection of a beam traveling toward the interface of the two media can be altered such that the intensity of the light entering the second medium approaches zero and substantially all of the light is reflected at the interface. Conversely, for any two transparent media, there is a critical angle of incidence at their interface at or below which substantially all of the incident light will be reflected. This phenomenon, known as total internal reflection, is applied in choosing the refractive indices of the core and the cladding in optical fibers so that light may propagate through the core of the fiber with minimal power loss.
Many other factors affect the propagation of light through the fiber optic core, including the dimensions of the core and the cladding, the wavelength of the light, the magnetic field vectors of the light and electrical field vectors of the light. In addition, many of the physical laws used to determine the ideal propagation of light through a waveguide (optical fiber) assume an “ideal” waveguide, i.e. a straight waveguide with perfect symmetry and no imperfections. For example, the diameter of the core will determine whether the optical fiber is “single mode” or “multimode”. The terms single mode and multimode refer to the dimensional orientation of rays propagating through the fiber. Single mode fibers have a core with a relatively small diameter (2-12 microns) and support only one mode of propagation, axial. Multimode fibers have a core with a relatively large diameter (25-100 microns) and permit non-axial rays or modes to propagate through the core. The so-called single mode fibers are actually two mode fibers in the sense that there are two different states of optical polarization that can be propagated through the core. In an ideal, straight, imperfection-free fiber with perfect circular symmetry, the propagation velocity of light is independent of the direction of polarization.
A fiber with an elliptical core will have two preferred directions of polarization (along the major axis and along the minor axis). Linearly polarized light injected into the fiber at any other direction of polarization will propagate in two separate modes that travel at slightly different velocities. This type of fiber is said to have a “modal birefringence”. In a real fiber of this type, even ideally polarized light will couple into the other mode due to imperfections in the core-cladding interface, index of refraction fluctuations, and other mechanisms. Static and dynamic changes in polarization may occur along the entire length of the fiber. Over a given distance, the phases of the two modes will pass through an entire cycle of being in phase and out of phase. This distance is known as the “beat length”. A long beat length is associated with a small birefringence and a short beat length is associated with a large birefringence. Birefringent optical fibers are also known as “polarization preserving fibers” or “polarization maintaining (PM) fibers”. Birefringence is achieved by providing a core with an elliptical cross section or by providing a circular core with a cladding which induces stress on the core. For example, the cladding may be provided with two parallel stress members having longitudinal axes which lie in the same plane as the axis of the core.
Fiber optic sensors employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. The primary advantages of fiber optic sensors include their ability to be lightweight, very small, passive, energy efficient, rugged, and immune to electromagnetic interference. In addition, fiber optic sensors have the potential for very high sensitivity, large dynamic range, and wide bandwidth. Further, a certain class of fiber sensors may be distributed or multiplexed along a length of fiber.
One type of fiber optic sensor is a side hole fiber optic pressure sensor that has two parallel holes which run the length of the fiber and are parallel to the core. The axes of the holes and the core lie in a common plane. This geometry results in converting external hydrostatic pressure into anisotropic stress at the core thereby inducing birefringence. Jansen and Dabkiewicz in an article entitled “High Pressure Fiber Optic Sensor with Side Hole Fiber”, published in SPIE Proceedings, Fiber Optic Sensors II, Vol. 798, pp. 56-60, 1987 describe such a structure. Changes in temperature also affect the birefringence of the core. However, the sensitivity of the side hole fiber sensor to pressure is significantly greater than its sensitivity to temperature. Thus, the side hole fiber optic pressure sensor can be used effectively in applications where temperature variations are minimal. In applications where both temperature and pressure are variable, complex measures must be taken to compensate for the effects of temperature on the birefringence of the sensor and the resulting pressure measurement. Moreover, the relative insensitivity of the side hole fiber optic pressure sensor to temperature makes it unsuitable for measuring temperature. Thus, a separate and distinct temperature sensor co-located with the side hole fiber optic pressure sensor is typically employed for this purpose.
Another type of fiber optic sensor utilizes a fiber Bragg grating. The fiber Bragg grating is formed in the core of the optical fiber by doping an optical fiber with a material such as germanium and then exposing the side of the fiber to an interference pattern to produce sinusoidal variations in the refractive index of the core. Two presently known methods of providing the interference pattern are by holographic imaging and by phase mask grating. Details of the methodology for manufacturing such fiber Bragg gratings are discussed in U.S. Pat. No. 5,380,995. The center wavelength of the spectral envelope reflected by the fiber Bragg grating changes linearly with temperature and strain. Thus, such changes can be measured to derive temperature and strain in the environment of the sensor as described in U.S. Pat. No. 5,380,995.
The fiber Bragg grating can also be formed as part of the core of a side hole fiber optic pressure sensor as described in U.S. Pat. No. 5,841,131. In this structure, the wavelengths of the peaks (and their shift relative to each other) in the spectral envelope reflected by the Bragg grating will change based upon the hydrostatic pressure applied to the sensor. Thus, such changes can be measured to derive pressure in the environment of the sensor. Similar to the side hole fiber optic pressure sensor, temperature affects the birefringence of the core and it is difficult to separate the pressure-related and the temperature-related contributions to the overall wavelength shift in the reflected spectral envelope. Thus, in certain applications where both temperature and pressure are variable, complex measures must be taken to compensate for the effects of temperature on the birefringence. Such complex measures are described by Chmielewska et al. in the article entitled “Measurement of pressure and temperature sensitivities of a Bragg grating imprinted in a highly birefringent side hole fiber,” Applied Optics, Vol. 42, No. 21, November, 2003. In this paper, the reflected spectrum is analyzed to identify the wavelength shift at two orthogonal polarization modes (LP01x, LP01y). One of the modes (LP01x) is highly sensitive to temperature yet insensitive to pressure. The other mode (LP01y) is sensitive to both temperature and pressure. These characteristics can be exploited to derive simultaneous temperature and pressure measurements by interrogation of the wavelength shifts at the two polarization modes. However, such compensation schemes are difficult and costly to implement for different applications and installations. Additionally, the wavelength sensitivity to pressure in this approach is quite small (about 1 picometer/18 psi (1 picometer/1.27 kg per square cm)), and it is difficult to achieve better than 0.1 picometer (pm) wavelength resolution with current optical technology. Therefore, it is very difficult to use this approach in most applications in which a high resolution pressure measurement is required. A mechanical amplifier can be applied to the fiber grating in order to increase its pressure sensitivity, but this makes it more difficult to manufacture and creates stability and repeatability problems.